Recently, in the field of color image forming apparatus utilizing an electrophotography scheme or an ink jet scheme, such as color printers and color copying machines or the like, enhancement in quality of output images has been demanded. Especially, density gradation and its stability are crucial factors that greatly affect how image quality is judged.
However, in the color image forming apparatus, changes in conditions of some portions in the apparatus caused by changes in environmental conditions or by long time use of the apparatus bring about changes in density of output images. Especially, in the case of the color image forming apparatus utilizing an electrophotography scheme, even slight changes in environmental conditions cause a change in density so as to deteriorate color balance. So it is necessary to provide some means for maintaining constant density-tone characteristics. Therefore, such apparatus are generally provided with tone correcting means, such as processing conditions including a plurality of exposure quantities and developing biases and a look up table (LUT), and processing conditions or values for gradation correction that would be optimum at that time are to be selected. In addition, in order that constant density-tone characteristics can be obtained even if changes in conditions occur in some portions of the apparatus, toner patches for density detection are formed on an intermediate transferring member or a drum etc. with toners of respective colors. The density of these toner patches (which have not fixed yet) is detected by a density detection sensor for unfixed toner (hereinafter referred to as a density sensor), and the result of the detection is used as feedback to a processing condition such as an exposure value or a developing bias etc so as to control the density so that stable images would be obtained.
However, in the above-mentioned density control method using the density sensor, since the patches are formed on an intermediate transferring member or a drum so as to be detected, changes in the color balance of an image caused by subsequent processing such as transferring to a transferring material or fixing are not controlled. Actually, changes in the color balance are caused also by application of heat and pressure during fixing or depending on transferring efficiency in the transfer of a toner image onto a transferring material. Such changes cannot be controlled by the above-mentioned density control using the density sensor.
In view of the above situation, such an color image forming apparatus having a sensor (which will be referred to as a color sensor hereinafter) for detecting the color of a patch formed on a transferring material is conceived in which a gray patch formed by black (K) and a process gray patch formed by mixing cyan (C), magenta (M) and yellow (Y) are produced, and after fixing them, the colors of those two patches are compared relatively, so that a mixing ratio of cyan, magenta and yellow with which the process gray patch would become achromatic can be determined.
In this color image forming apparatus, the result of the detection may be used as feedback to an exposure value or a processing condition of an image forming section, a color matching table for converting an RGB signal in the image processing section into a color reproduction gamut (area) of the color image forming apparatus, a color separation table for converting an RGB signal to a CMYK signal, or a calibration table for correcting density-gradation characteristics. Thus, the color image forming apparatus can control the density or chromaticity of a final output image that has been formed on a transferring material.
A control process similar to the above can also be performed by detecting an image output from the color image forming apparatus using an external image reading apparatus or color meter and densitometer. However the above-described control process using the color sensor is more advantageous in that the control is completed in the printer itself. The color sensor is arranged to include as light emitting elements, for example, three or more light sources that have light emission spectrums different from each other (e.g. red (R), green (G), and blue (B) etc.). Alternatively, the light emitting elements of the color sensor may be comprised of a light source(s) emitting white (W) light that is provide with three or more filters that have spectral transmittances different from each other (e.g. red (R), green (G), and blue (B) etc.) formed thereon. With the above arrangements, three or more different outputs, such as R, G and B outputs, can be obtained from the color sensor.
In the control process using the color sensor as described above, it is necessary to form patches on a transferring material, whereby the transfer material and toners are consumed. Therefore that control process cannot be performed frequently. So there exists a need in the art to perform effective controlling of density or chromaticity with a reduced frequency of the control process.
On the other hand, in order to enable the above-mentioned correction for realizing an achromatic process gray, means for obtaining a ratio of cyan, magenta and yellow with which the process gray becomes achromatic is required.